Antimicrobial Susceptibility of Glaesserella parasuis to Macrolides and Characterization of erm(T)-Carrying Mobile Elements on Chromosome

<i>Glaesserella parasuis</i> is the etiological agent of Glässer's disease, which causes high morbidity and mortality in pigs worldwide. Macrolide resistance poses an urgent threat to their treatment, as macrolides are widely used for preventing and treating <i>G. parasuis</i> infections. Here, we determined the susceptibilities to five macrolides and characterized the genetic markers of macrolide resistance. The antimicrobial susceptibility of 117 <i>G. parasuis</i> isolates to erythromycin, tulathromycin, gamithromycin, tylosin, and tilmicosin was evaluated using broth microdilution method. Erythromycin-resistant isolates were sequenced using whole-genome sequencing. Further analysis of these sequences revealed the genetic basis of macrolide resistance in <i>G. parasuis</i>. Our results show that most <i>G. parasuis</i> isolates remained susceptible to the macrolide drugs. For commonly used agents (e.g., tylosin and tilmicosin), elevated minimum inhibitory concentrations (MICs) were observed, whereas for the newer macrolides (e.g., tulathromycin and gamithromycin), the MICs remained almost unchanged. The macrolide resistance gene <i>erm</i>(T) and the A2059G mutation in 23S rRNA were detected in the current study. To the best of our knowledge, integrative and conjugative element (ICE)-borne <i>erm</i>(T) in <i>G. parasuis</i> is reported for the first time in this study. Taken together, these results provide insights into the susceptibility of <i>G. parasuis</i> to macrolides. The presence of <i>erm</i>(T) on ICEs may facilitate its transfer, reducing the effectiveness of macrolide treatment.